Does central fatigue exist under low-frequency stimulation of a low fatigue-resistant muscle?

Abstract

The aim of the present study was to determine whether central fatigue occurs when fatigue is electrically induced in the abductor pollicis brevis muscle. Three series of 17 trains (30 Hz, 450 μs, 4 s on/6 s off, at the maximal tolerated intensity) were used to fatigue the muscle. Neuromuscular tests consisting of electrically evoked and voluntary contractions were performed before and after every 17-train series. Both the force induced by the stimulation trains and maximal voluntary force generation capacity significantly decreased throughout the protocol (-27 and -20%, respectively, at the end of the protocol, P < 0.001). These decreases were accompanied by failure in muscle excitability (P < 0.01), as assessed by the muscle compound action potential (M-wave or Mmax), leading to significant impairment in the muscle contractile properties (P < 0.05), as assessed by the muscle mechanical response (Pt). Central fatigue indices (level of activation, RMS/Mmax and H reflex) were not significantly changed at any point in the protocol. This gives evidence of preserved motor command reaching the motor neurons and preserved spinal excitability. The results indicate that this low-frequency stimulation protocol entails purely peripheral fatigue development when applied to a low fatigue-resistant muscle.

Initial Electrical Stimulation Frequency and Cramp Threshold Frequency and Force

“…Other 8 researchers have observed that fatigue can be induced after low-frequency electrical stimulation. Papaiordanidou et al 8 noted decreases in MVIC force and maximal M-wave amplitude of the abductor pollicis after 17 trains of 30-Hz stimulation with a pulse duration of 450 ls and a duty cycle of 40% (4 seconds on, 6 seconds off). These observations suggest that muscle excitability was lowered after stimulation, and the authors inferred the presence of peripheral fatigue.…”

“…Whereas beginning at lower initial frequencies prevents overshooting the TF of an individual, applying many subthreshold electrical stimuli to a small muscle, such as the flexor hallucis brevis, might induce fatigue. Lowfrequency electrical stimulation can cause fatigue, 8 and we have observed that inducing 2 cramps within 1 minute of each other resulted in a second cramp that was weaker and lasted a shorter duration. 1 Moreover, fatigue has been observed to cause delays in rates of relaxation, thereby increasing the likelihood of fused tetani, 9 and might increase the excitability of the a motor-neuron pool by increasing muscle spindle activity.…”

Mechanisms of Fatigue and Task Failure Induced By Sustained Submaximal Contractions

“…Once the optimal intensity was found, it was further increased by 10%. The relatively smaU 10% increment to ensure supramaximal stimulation intensity has been used in previous studies (13,20) and was chosen in the present work to limit the discomfort associated with the stimulation. Then this intensity was kept constant throughout the experiment for each subject.…”

Comparison of neuromuscular adjustments associated with sustained isometric contractions of four different muscle groups

“…This intensity was then increased by 10% and kept constant throughout the experiment. Such supramaximal stimulation intensity, as used in previous studies (46,53,56), limits the discomfort associated with the stimulation compared with higher supramaximal levels. Pulse width was set at 0.1 ms for the upper limbs (68) and at 1 ms for the lower limbs (53,57).…”